Recently, with increasing size of vehicles, there is also an increasing need for a large passenger car tire. In response to user's request for improvement of tire performance and appearance, a so-called “extra load tire” setting is increasing, in which a usage under high inner pressure and high load is possible.
Examples of a performance which is demanded for a belt cord or a belt treat for such an extra load tire or a tire for a large passenger car include a resistance represented by regulations related to plunger energy or belt edge separation resistance (belt edge separation; hereinafter, also referred to as “BES”).
Among these, although the plunger energy is determined by the design of a whole tire, and not determined only by changing the material of the belt, increase in belt strength results in increase in the plunger energy since the belt strength also has a large influence. The use of cross belt layer for a pneumatic tire generates an interlayer shear strain between two belt layers to thereby generate a BES. In particular, since a tire for a large passenger car or an extra load tire is assumed to be used under severe environments such as under high inner pressure or high load, shear strain at the edge of a belt layer is higher than that of normal tires, and therefore, when the same material or the same structure as those of normal tires is employed, a BES is likely to occur, which is problematic.
In general, a steel cord for a tire needs to have an excellent rubber penetration property and to be lightweight and inexpensive. The former, the rubber penetration property is important for preventing the progression of corrosion in the cord when the tire is cut due to a nail or the like. Regarding the latter, from the viewpoint of considering global environment, for contributing to “REDUCE”, one of “3R”, the lighter the better, which means small amount of material to be used, leading to resource saving. When an expensive material is used, it becomes difficult for the tire to be prevailed in public, and therefore the cost needs to be low.
As an improvement technology relating to a steel cord as a reinforcing member for a belt, Patent Document 1 discloses a steel cord for reinforcing a rubber article comprising a core composed of two wires and having an average twisting pitch of at least 30 mm and a sheath composed of six wires which is wound around the core, wherein the ratio of the diameter ds of the wire of the sheath with respect to the diameter dc of the wire constituting the core ((ds/dc)×100) is in a range of 58.0% <ds/dc<161.5%. Patent Document 2 discloses a steel cord comprising a core composed of two filaments having the same diameter which are arranged in parallel without being twisted together and a sheath composed of six filaments having the same diameter as that of the core filament which has been wound around the core, wherein there is a gap between the sheath filaments and an arrangement of the sheath filaments in which at least two gaps have a spacing of ⅓d or more with respect to the diameter of the filament d appears at least once per one twist length of the sheath.
Further, Patent Document 3 discloses a steel cord for reinforcing a rubber article comprising a core in which two core wires are arranged in parallel without being twisted together and 5 to 7 sheath wires which are twisted together around the core, wherein the cross-section shape of the cord is substantially an ellipse, the diameter dc (mm) of the cord wire satisfies the range of 0.05≦dc<0.26, the diameter ds (mm) of the sheath wire satisfies the range of 0.05≦ds<0.26 and the ratio of the diameter dc of the core wire to the diameter ds of the sheath wire dc/ds satisfies the relationship of 0.7<dc/ds<1. Still further, Patent Document 4 discloses a pneumatic radial tire wherein, as a steel cord to be applied to a cross belt layer, one having a single ply structure or a core-single layer sheath structure composed of 6 to 10 steel wires having a wire diameter of 0.10 to 0.20 mm is employed; the end count thereof is 40/50 mm or more; and the distance between adjacent steel cords in the belt layer is 0.3 mm or more.